As is well known, in a Sequential Color and Memory Television System, hereinafter referred to as SECAM, the frequency modulated sub-carrier is pre-emphasized to reduce the relative amplitude of the central frequencies. Correspondingly, this predistortion of the spectrum is equalized by a filter with a bell-shaped response used at the reception end before demodulation. This filter, which separates the chrominance from the luminance and synchronization components of the SECAM signal is, most generally, a parallel LRC network (inductance, resistance and capacitance) having a response maximized at about 4.286 MHz. The filter, because of its swept frequency response is conventionally known as a "Bell" filter and suffers from two principle deficiencies. The first deficiency is that whenever synchronization or luminance components of the SECAM signal are passed therethrough, a coherent interfering signal is injected which effects operation of limiters used in SECAM decoders. The second deficiency is that when the chrominance portion of the signal is turned on (or off), the result of this switching transient combines with the steady state to create a spurious frequency modulation. This transient causes a shift in the zero crossings of the subcarrier toward a higher or a lower frequency which is subsequently limited and decoded.
These above described deficiencies therefore make color sequence determination from the white lead-in very difficult. In addition to this most difficult problem, these deficiencies also inhibit parameter measurements of the SECAM signals during horizontal blanking.